1. Field of the Invention
The present invention relates to an apparatus for controlling exposure quantity in the photolithography process to manufacture semiconductor devices. More specifically the invention relates to an apparatus for controlling the illumination energy quantity to a suitable sensitive substrate, for example, in an exposure apparatus using a pulse laser such as an eximer laser as exposure light, and to an exposure apparatus provided with such an exposure control apparatus.
2. Description of the Prior Art
In the prior art, since laser light generally has a variation of intensity of about .+-.10% between pulse and a phenomenon of lower laser output density exists for a short period or for a long period, control of exposure quantity in an exposure apparatus using a pulse laser as a light source has been performed in a manner that light quantity in each pulse is detected and integrated and the laser emission is performed until the integration result becomes a desired value.
In general, lower phenomenon of the laser output density is conspicuous in the case of using a gas laser, because deterioration of laser gas within a laser chamber is accompanied by the lowering of the laser output. It is usual that, when the inner gas has deteriorated in a gas laser and the laser output is lowered, the discharge applied voltage of the device or the like is adjusted and the discharge energy is increased, or in some cases the inner gas is partially exchanged by a fresh laser gas and the lowering of the laser output is reduced. However, the control of the desired exposure quantity with fixed pulse number is difficult.
Under these circumstances, a control apparatus requiring the exposure quantity control with a higher accuracy such as in exposure control in an exposure apparatus for manufacturing semiconductor devices is disclosed in Japanese patent application laid-open No. SHO-60-169136.
In this apparatus, the exposure energy to be supplied to a sensitive substrate (such as wafer with resist) is divided into two stages, pre-exposure giving the exposure energy slightly less than the proper exposure quantity and revisional exposure giving the residual exposure energy required. Thereby variation of the exposure energy as a whole is controlled. When exposure of one shot is performed by plural pulses, the exposure quantity is controlled by the final pulse with the energy quantity less than that of the other pulse, thereby the optimum exposure quantity can be obtained. In this connection, one shot means that the exposure energy is illuminated through a mask to the whole wafer in the case of a lump exposure system, and that the exposure energy is illuminated to one partial region on the wafer in the case of step-and-repeat system (described later).
In step-and-repeat system adopted in the exposure apparatus for manufacturing semiconductor devices in recent years, in order to expose one wafer, the region to be exposed is divided into a plurality of exposure regions and the exposure is performed in each exposure region. When the exposure in one exposure region is finished the exposure is moved to next exposure region and performed again, and this operation is repeated thereby the exposure of the whole surface of the wafer is finally performed.
Consequently, in order to increase the production amount of the semiconductor devices per unit time, it is important that the moving time between the exposure regions is made as short as possible and the exposure time in each exposure region is made as short as possible. Therefore, even when the exposure is performed by means disclosed in Japanese patent application No. SHO-60-169136, it is necessary that the pre-exposure is performed by energy as large as possible and the time of the revisional exposure is made short.
In the method disclosed in the Japanese patent application laid-open No. SHO-60-169136, however, since feedback control does not exist for error (variation) of the energy quantity contained in the final pulse, a disadvantage exists in that the exposure quantity is not controlled properly and the suitable exposure cannot be performed.
Since the setting energy of the final pulse becomes the value of the proper exposure quantity subtracted by the pre-exposure quantity, problem exists in that the dynamic range of means for setting the exposure quantity of the final pulse must be made large, thereby the apparatus is complicated and high control accuracy cannot be obtained.
Japanese patent application laid-open No. SHO-63-81862 discloses a method of obtaining the proper exposure quantity using the revisional exposure as plural pulses. However, also in this case, the problem is not eliminated that the exposure time becomes long and the dynamic range of the exposure quantity setting means becomes large.
Further in a method of adjusting the exposure quantity by the revisional exposure as above described, since the energy quantity of one pulse and the exposure pulse number during the revisional exposure are dependent on the integrated exposure quantity when the pre-exposure is finished, a variation exists in that these values do not become constant in each shot. That is, when the exposure energy source is a laser light source, unevenness of intensity of illumination called speckle may be produced in the exposure surface due to coherency of the laser light, and the above-mentioned light quantity control method cannot reduce this speckle effectively. This problem will be described as follows.
Since the speckle has a serious influence on the control of the pattern line width in the photolithography process during manufacturing the semiconductor devices, as taught in U.S. Pat. No. 4,619,508 for example, the speckle pattern (or interference pattern) may be moved at the emission of each pulse (the laser light may be oscillated) thereby the speckle may be smoothed. However, in order that method in U.S. Pat. No. 4,619,508 and method in Japanese patent application laid-open No. SHO-63-81882 are combined and both the optimization of the exposure quantity and the reduction of the speckle are performed, as disclosed in U.S. Pat. No. 4,619,508, the light source image (laser spot) on the pupil plane of the projection lens must be distributed as uniformly as possible within the pupil plane. In order to realize this, it is necessary that the point number of the laser spots in the pupil surface be made constant and the exposure if performed at the pattern number of integer times of the point number of the laser spots and the laser intensity is held constant during the exposure. However, in the method of Japanese patent application laid-open No. SHO-63-81882 as above described, only the region corresponding to the revisional exposure state has lower integrated energy than that of other region, and as a result the speckle cannot be reduced sufficiently.